Small bolt pretensioner proposal

ASML – Small Bolt Pretensioner
Background
Whether you are reading this on a computer, on a smart phone, on a tablet, or on any other device, you are using ASML's lithography products right now. In lithography, an image on a reticle is projected onto a wafer to etch the design into the wafer. Figure 1 shows a simplified version of some of the key components in the process. The bottom section shows the round wafer in a square carrier, while the middle section shows the rectangular reticle. The top circular section is part of the optical system.

Improvement in the lithographic process is so important because as lithography improves, so does the speed and capability of the microchips that are created from the process. Following "Moore's Law", advances in lithography systems, such as the one shown in Figure 2, have allowed chip manufacturers to continue to shrink feature sizes, with the current state of the art comes the ability to manufacture feature sizes in the range of 22 nanometers, or about the size of 10 silicon atoms.

With feature sizes in that range, tolerance budgets are pressed to the atomic level. All components and processes contributing to the precision of the equipment must be optimized.


For non-precision applications, torque wrenches / torque drivers, as shown in figure 3, can be used to create somewhat repeatable performance in bolt tightening. However, this method has the following disadvantages:

1. Poor accuracy due to dependence on friction coefficients of components. For non-lubricated vacuum components, this depends highly on surface quality and particulate contamination and is impractical to know accurately and reliably.
2. Large displacements of the clamped part due to friction between screw head and clamped part.
3. Introduces surface defects on the clamped part, making subsequent actions less accurate and reliable

Figure 3 – Bolt / nut torque application
For larger screws, the use of hydraulic tensioners, shown in Figure 4, offers greater precision. However, no commercial product of this type exists for the small screw sizes used in ASML's precision mechatronic assemblies. Also, hydraulic devices are incompatible with the clean room requirements needed for semiconductor manufacturing.

Figure 4 – Hydraulic Bolt Pretensioner



Problem Statement
For this project, students are challenged to develop a method to assemble a bolted connection for small screws (M1.6-M3) for subassemblies such as Figure 5 that meets the following functional requirements:

1. Applies an accurate and known bolt tension (+/- 5%).
2. Minimal (5-10um) transverse alignment distortion of the clamped part with respect to the base part.
3. Applicable to screws spaced 4 - 5.5mm (M1.6) to 8 mm (M3) apart.
4. Does not cross contaminate high vacuum compatible components, e.g. no lubricants allowed; minimize any effects from sliding surfaces / scratches which would create particles.

Figure 5 – Sample Subassembly
  
Students will be expected to design their system in CAD (in their program of choice), analyze its capabilities using appropriate tools (traditional calculations, FEA, etc.), build their system, and test its performance. Test should use appropriate instrumentation and number of samples to verify the primary requirements for accuracy are met

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Contact Matt Ulinski, mu25@cornell.edu